Mattress and box spring won’t stay in one place overnight. Any recommendations?

Buy rolls of the rubber shelf lining stuff at Wally World, that is what I use.

We put a big roll of anti slip material that goes under rugs between box spring an matress. It still moves a tad but not as bad

Could use strips of Velcro too. They come with sticky backing. Another idea is getting rid of the boxspring if possible. You can put slats from one side of the frame to the other then a couple of 1/4 inch plywood. Put the mattress on top of that. Helps getting out of bed easier and lower to the floor if the person falls out of bed.

If you mean that they slide apart, you could try the rubbery type placemats or the rubber sheets that go under rugs.